1. Field of the Invention
The present invention relates to a linear-motion bearing unit, and more specifically to such a bearing of type incorporating groups of recirculating balls for a minimum of friction. The linear bearing in accordance with the present invention is suitable for use in the table saddle of a machine tool or the sliding section of a machining center or conveyor where one part is to be moved back and forth relative to another.
2. Description of the Prior Art
A typical conventional linear-motion bearing unit will be explained hereinunder with specific reference to FIG. 17. This conventional linear-motion bearing unit has a bearing body 101 having an inversed U-shaped cross-section and mounted on a track shaft 103 through the intermediary of steel balls 102,102, in such a manner that it can move relative to the track shaft 103 in the longitudinal direction thereof, as shown in FIG. 17. This conventional bearing unit also includes longitudinal loaded ball rolling grooves 101a,101a which are formed on the inner side of the bearing body 101, and longitudinal loaded ball rolling grooves 103a,103a formed on each side of the track shaft, such that the loaded ball rolling grooves 101a,101a face the loaded ball rolling groove 103a,103a on each side of the track shaft 103. The bearing unit further has longitudinal non-loaded ball rolling bores 105,105 which are connected to the loaded ball rolling grooves 101a, 101a; 103a, 103a through ball turning passages provided in end plates (not shown) mounted on each both end of the bearing body. The non-loaded ball rolling bores 105, 105 are formed in each of skirts 104, 104 of the bearing body 101. A multiplicity of steel balls 102, 102 are charged in the ball rolling passages constituted by the loaded-ball rolling grooves 101a, 101a; 103a, 103a and the non-loaded ball rolling bores 105, 105 so that the steel balls can circulate in respective ball rolling passages when the bearing body 101 moves relative to the track shaft 103 in its axial direction. A retainer 106 having a pair of longitudinal slots 106a, 106a is disposed on each side of the bearing body 101. The steel balls 102, 102 rolling along the loaded ball rolling grooves 101a, 101a; 103a, 103a of the bearing body 101 and the track shaft 101 are guided by the slots 106a, 106a of the retainer which also serve to prevent the steel balls 102, 102 from coming off.
In such a conventional bearing unit, the non-loaded ball rolling bores 105, 105 are provided by being drilled in the longitudinal direction in the solid skirts 104, 104. In consequence, if the bearing body 101 is very long, it is difficult to conduct accurate drilling owing to the deflection of the drill which is used, or for other reasons. The drilling may alternatively be started from each end of the bearing body 101 and proceed all the way to the central portion thereof where a final hole is drilled. In that case, however, a step could easily be created at the central portion, and it may be compensated for by redrilling the hole or by reaming it. This requires a large amount of time and labour, resulting in an increase in cost.
This arrangement of a conventional bearing unit limits the length of the bearing body 101, so that the lengths of the loaded ball rolling grooves 101a, 101a; 103a, 103a are inevitably limited, and it is therefore difficult to increase the rated load of the ball bearing by increasing the lengths of the loaded ball rolling grooves.
The bearing body 101 of this conventional bearing unit is made of a steel and therefore generates a relatively high level of noise during rolling of the steel balls 102, 102 along the non-loaded ball holes 105, 105.